1. Field of the Invention
The present invention relates to an overrunning clutch system, and more particularly to an overrunning clutch system, for example, suitable for the engine starter used for an agricultural machine such as a lawn mower, a sprayer or the like.
2. Description of Related Art
Referring now specifically to FIG. 4, illustrating a prior art overrunning clutch system, there is provided a starter motor 52 having an output shaft on which a drive gear 54 is mounted. An idle gearing 56 has a large gear 56a and a small gear 56b, which are respectively meshed with the drive gear 54 and a reduction gear 58.
The reduction gear 58 is fixedly mounted on a drive shaft or cylindrical transmission sleeve 60 coaxially fitted around an overrunning clutch 64 which has a shell type construction and is disposed between the transmission sleeve 60 and the crankshaft 62.
With this arrangement, drive power is transmitted from the starter motor 52 to the transmission sleeve 60 with the speed of rotation being reduced through the gears.
As shown in FIGS. 5 and 6, the overrunning clutch 64 includes a shell 66, a retainer 68, and a plurality of rollers 70. The shell 66 has a cylindrical portion 66a and a pair of flanges 66b extending radially and inwardly from the axial opposite ends of the cylindrical portion 66a.
Cam surfaces 66c corresponding in number to the rollers 70 are formed in the inner periphery of the cylindrical portion 66a. The retainer 68 is made of a synthetic resin and includes a pair of opposite annular rings 68a, and a plurality of column portions 68b extending axially between these rings 68a. A plurality of roller pockets 68c are defined between adjacent column portions 68b, wherein the rollers 70 are correspondingly rollingly disposed. Each of the column portions 68b in the retainer 68 has an integral spring 68d which takes the shape of a bifurcated tongue adapted to urge the roller 70 in the roller pocket 68b in the direction that the roller 70 is locked between the cam surface 66c and the crankshaft 62. The shell 66 of the overrunning clutch 64 thus constructed is pressed in the transmission sleeve 60.
The operation of the above-mentioned overrunning clutch system is as follows.
To crank up an engine, not shown, when the starter motor 52 is energized to rotate the sleeve 60 in the direction of an arrow a2 through each of the gears 54, 56, and 58, the shell 66 and the retainer 68 are rotated in the same direction. The rollers 70 urged by the springs 68d are then moved in the direction that the space between the cam surfaces 66c and the crankshaft 62 is narrower, and the cam surfaces 66c and the crankshaft 62 are finally locked.
In this manner, the rotation of the shell 66 is transmitted through the rollers 70 to the crankshaft 62, such that the crankshaft 62 is rotated in the direction of an arrow b2 so as to crank up the engine connected thereto.
When the engine is thus cranked up, the starter motor 52 is stopped, while the crankshaft 62 is rotated at a high speed in the direction of the arrow b2 by means of the engine. At this time, the transmission sleeve 60 is stopped since it is connected through the reduction gear 58, the idle gearing 56, and the drive gear 54 to the starter motor 52. Hence the shell 66 and the retainer 68 secured respectively to the transmission sleeve 60 are also stopped.
In such a state, by frictional force applied from the crankshaft 62 to the rollers 70, the rollers 70 are pushed against the springs 68d in the direction of the arrow b2 in which the space between the cam surfaces 66c and the crankshaft 62 is wider, whereby the rollers 70 are enabled to freely rotate within the roller pockets 68c, thereby causing the cam surfaces 66c and the crankshaft 62 to be unlocked, so that power is no longer transmitted from the crankshaft 62 to the shell 66 and finally the starter motor 52.
It has been stated that when the crankshaft 62 is rotated by the engine, the rollers 70 are moved in the direction in which the space between the cam surfaces 66c and the crankshaft 62 is wider.
Actually, although the rollers 70 are moved in the direction of the wide space between the cam surfaces 66c and the crankshaft 62 so as to race immediately after they come into contact with the crankshaft 62, they are moved in the direction of the narrow space therebetween by the action of springs 68d as soon as a push force from the crankshaft 62 is no longer applied, thereby being brought into contact with the crankshaft 62.
Such an intermittent movement in which the rollers 70 are contacted with and separated from the crankshaft 62, is so-called a "dancing phenomenon" of the rollers 70. When the rollers 70 in the phenomenon are moved in such a direction as to lock the cam surfaces 66 and the crankshaft 62, they become worn and heated by means of the frictional force applied from the crankshaft 62, such that the crankshaft 20 is prevented from smoothly rotating. Further, friction between the rollers 70 and the crankshaft 62 generates heat, thereby resulting in an increase in the temperature of the parts comprising the rollers 70, the crankshaft 62, and the retainer 68, and when the temperature is raised above the predetermined level, seizing of the parts may occur.
If seizing occurs, the crankshaft 62 and the shell 66 are locked to give an impact or to cause damage to the overrunning clutch system. The retainer 68 is easily damaged due to the frictional heat, particularly in the case where the retainer 68 with a tongue-like spring 68d is made of a synthetic resin.
Furthermore, to hold the rollers 70 in the wide space so that they are not moved in the direction of the narrow space between the cam surfaces 66c and the crankshaft 62 against the biasing force of the springs 68d and thus to suppress the generation of the above frictional heat wherever possible, it is considered that the springs 68d be formed to have a smaller biasing force.
With the springs 68d having the smaller biasing force, however, it is difficult for the rollers 70 to firmly lock between the cam surface 66c and the crankshaft 22 so as to transmit rotation from the sleeve 60 to the crankshaft 62 when the engine is started up.
FIG. 7 is a longitudinal sectional side view of an overrunning clutch system proposed by U.S. Pat. No. 5,117,954 corresponding to Japanese Application Number 2-207383, filed on Jul. 30, 1991, to solve the above "dancing phenomenon" of the rollers 70. In FIG. 7, the same reference numerals are provided for the elements corresponding to those in the overrunning clutch system shown in FIGS. 4 through 6. The overrunning clutch system of FIG. 7 basically includes a starter motor 52, a drive gear 54, an idle gearing 56, a reduction gear 58, a crankshaft 62, an overrunning clutch 64, a shell 66, a retainer 68, and a plurality of rollers 70.
The crankshaft 62 has a large diameter portion 62a, a small diameter portion 62b, and an attaching portion or threaded portion 62c. One flange 66b at the left side of the shell 66 is extended radially and inwardly, being spline-engaged with a crankshaft 62. An annular abutment plate 72 is held in the inner side of the flange 66b and pressed against a step 62c formed with the one end surface of the large diameter portion 62a of the crankshaft 62.
A C-shaped ring 74 is mounted adjacent the annular abutment plate 72 in an annular groove 62d formed with the one end surface of the small diameter portion 62b.
With this arrangement, when the starter motor 52 is driven, the crankshaft 62 is rotated so as to start up the engine in the same manner as in the above system of FIGS. 4 to 6, such that the rollers 70 are moved in the direction where the space between cam surfaces (not shown) and the crankshaft 62 is wider, and thereafter is left under a freely racing state in the wide space. The freely racing rollers 70 in the wide space are forced radially and outwardly by centrifugal force.
After being moved in the wide space direction in the overrunning clutch 64, the rollers 70 are not moved by the action of springs (not shown) in the direction where the space between the cam surfaces and the crankshaft 62 is narrower, which results in that the "dancing phenomenon" of the rollers 70 is not caused.
In the overrunning clutch system shown in FIG. 7, however, since the C-shaped ring 74 is set to be a smaller axial width than the annular groove 62d on one end surface of the small diameter portion 62b, they become loose radially between the annular abutment plate 72 and the small diameter portion 62b. Such an axial looseness of the C-shaped ring 74 may cause skew of the rollers 70 and hence make the function of the overrunning clutch 64 very unstable.
Furthermore, since the outer periphery of the small diameter portion 62b of the crankshaft 62 functions as a guideway for the transmission sleeve 62, expensive laborous replacement of the entire crankshaft 62 may be required if the outer periphery thereof is damaged.